This application pertains to the art of control-safe operator machine control and more particularly to operator machine control using capacitive switches. The invention is particularly applicable to operation of potentially dangerous machinery such as that used in metal stamping, punch presses, and the like, and will be described with particular reference thereto. However, it will be appreciated that the invention has broader application as in any repetitively activated machinery for which minimization of operator hand pressure is advantageous.
A substantial portion of industrial machinery requires continuous interaction with human operators. Examples of machinery of this nature includes metal forming equipment, such as punch presses or stamping machines, wherein an operator places a workpiece into the machine, activates the machine, and removes a completed workpiece. This process is repeated a substantial number of times in typical mass-production environments.
Presently, most metal forming equipment such as that described above, provides for two switches, both of which must be contacted to activate the machine. The switches are typically located and situated so as to discourage operators from circumventing the use of both operator hands to activate the machine. This provides for increased operator safety by minimizing an opportunity for a hand to be placed in the vicinity of the machinery during operation.
Employers are becoming increasingly aware of possible dangers associated with repetitive, mechanical actions of operators. One noted effect of continued and repetitive hand motions is that of carpel-tunnel syndrome. Switches typically employed in machinery control are contact, mechanically-operated push-button switches. Operation of these switches result in the repetitive forces on operator hand which may augment susceptibility to such maladies as carpel-tunnel syndrome.
Capacitive switches provide for electrical switching without mechanical switch toggling. Commonly available switches of this nature include capacitive proximity sensors marketed by Pepperl+Fuchs, Inc., the assignee of the subject application. Several varieties of these sensors are provided in the assignee's 1989 catalog, pages 112-123, the contents of which are incorporated herein by reference.
Common capacitive proximity sensors sense capacitive changes induced by certain objects placed proximate thereto. The human hand provides the capacitive changes for activation of the switches.
Capacitive switches are advantageously employed in a number of situations. They provide certain disadvantages when applied to uses in connection with control of machinery. Proximity sensors, such as capacitive switches, may be influenced to triggering by the presence of radio frequency interference ("RFI"). Such RFI is often found in industrial or manufacturing environments. The RFI may be induced by machinery as well as radio communication devices. Such spurious RFI may result in triggering of machinery for which capacitive proximity sensors have been used. If such a triggering coincides with placement of an operator's hand in the machinery, severe injury may result. Furthermore, conventional electronic control systems, particularly digital control systems, are succeptable to failure due to "stuck-at" faults in their control circuitry which may also lead to improper machine switching.
The present invention contemplates a new and improved switching system which addresses all of the above-referred problems, and others, and provides for machine operation with improved RFI immunity and minimized physical strain on the operator.